1. Field of the Invention
The present invention relates to an apparatus for projecting an image and an apparatus for observing an image and, more particularly, to those suitably applicable to liquid crystal projectors using a liquid crystal display element (liquid crystal panel) as an image display element and constructed to project an image obtained thereby through a projection lens, for example, onto a polarizing screen and to image observation systems constructed to permit observation of an enlarged and projected image from a screen of a computer, a picture of a video camera, or the like.
2. Related Background Art
A variety of proposals have been made heretofore as to the image projection devices (liquid crystal projectors) constructed to illuminate the liquid crystal panel with light from a light source, display an image on the liquid crystal panel, and enlarge and project an image based on transmitted or reflected light from the liquid crystal panel, through the projection lens onto the screen.
FIG. 17 is a schematic diagram of major part of a conventional image projection apparatus. In FIG. 17 reference numeral 101 designates a white light source. Numeral 102 designates a reflector. Numeral 103 represents a visible-light-transmitting filter for removing the components of light except for the visible light.
Numeral 104 indicates an integrator for yielding a uniform illumination area, which is comprised of fly""s eye lenses 104a, 104b each consisting of an array of lenses. Numeral 105 denotes an array of polarization converting elements for converting non-polarized light into linearly polarized light polarized in a predetermined direction of polarization, each element consisting of a polarization separating surface 105a, a reflecting surface 105b, and a half-wave plate 105c.
Numeral 106 represents a condenser lens. Numeral 107 designates a first dichroic mirror, 108 a second dichroic mirror, and 109a and 109b reflecting mirrors. Numeral 110 stands for a relay system for relaying the illumination light, which is comprised of relay lenses 110a, 110b and relay mirrors 110c, 110d. 
Symbols 111r, 111g, and 111b are condenser lenses for images (light beams) of the colors of R (Red), G (Green), and B (Blue), respectively. Symbols 112r, 112g, and 112b are image display elements for R, G, and B, respectively. Numeral 113 represents a cross dichroic prism DP for color composition. Numeral 114 stands for a projection lens.
The white light emitted from the white light source 101 is collected by the reflector 102 and then travels through the integrator 104, the polarization converting element array 105, and the condenser lens 106. After that, the light is separated into the color beams of R, G, and B light by the dichroic mirrors 107, 108. The first color light (B in the figure) is guided via the reflecting mirror 109b and condenser lens 111b to the image display element 112b, the second color light (G in the figure) is guided via the condenser lens 111g to the image display element 112g, and the third color light (R in the figure) is guided via the relay system 110 and condenser lens 111r to the image display element 112r. 
The color beams of R, G, and B, traveling through the image display elements 112b, 112g, and 112r and modulated according to image signals, are then combined into one by the cross dichroic prism DP 113, whereby the images displayed on the respective image display elements are enlarged and projected in a superimposed manner onto the screen (not illustrated) through the projection lens 114. A discharge lamp such as a metal halide lamp, a mercury lamp, or the like is used as the white light source.
FIG. 18 shows an example of spectral distribution of the white light source 101. From the white light having the continuous spectral distribution as illustrated, the dichroic mirrors DM1, DM2 create the three color beams of R, G, and B, for example, having respective spectral distributions as illustrated in FIG. 19.
In the conventional apparatus, these light beams are modulated by the respective image display elements 112r, 112g, 112b and thereafter combined by the cross dichroic prism DP. In order to avoid loss in light amount in the cross dichroic prism DP, dichroic films of the cross dichroic prism are designed so that light reflected thereby is s-polarized light components of red (R) and blue (B) while the light of green (G) transmitted by the dichroic films of the cross dichroic prism DP is a p-polarized light component.
The reason is that, from the characteristics of the dichroic films as illustrated in FIG. 20, a broader reflection band can be set in the case of the s-polarized light components being reflected by the dichroic films (BRs, RRs) and a broader transmission band can be set in the case of the p-polarized light component being transmitted by the dichroic films (GTp). This suppresses the loss of light amount in the dichroic prism due to the so-called incident angle characteristics of the dichroic films, which are variations in cut wavelengths of the dichroic films due to variations in angles of incidence of light to the dichroic films.
In order to realize this structure, where the polarization directions of the image beams emerging from the image display elements were as illustrated in FIG. 21, the apparatus was so constructed that a half-wave plate was placed in each of the three paths of the emergent beams and that the slow phase axis directions of the phase plates were set so as to make the polarization direction of G light perpendicular to the polarization direction of R and B light and so as to make the polarization direction of G light coincident with that of the p-polarized light with respect to the dichroic films of the dichroic prism DP.
In systems necessitating alignment of the polarization directions of projected light on the occasion of projection of image (for example, such as polarized image projection systems using the polarizing screen or stereoscopic image projection systems for projecting images for the right eye and for the left eye with beams having respective polarization directions different from each other), however, the polarization direction of G light has to be aligned with the polarization direction of R and B light by providing a polarizing means at an arbitrary position in the optical path from the dichroic prism to the polarizing screen or to the observer.
The reason is as follows. When the polarization direction of light reflected by the polarizing screen is set in parallel to the s-polarized light component of the dichroic prism, the color beam of green is absorbed. When the polarization direction of light reflected by the polarizing screen is set in parallel to the p-polarized light component of the dichroic prism, the color beams of red and blue are absorbed. This will result in failing to reproduce a correct color image.
It is then conceivable, for example, to convert the beams into the polarization directions inclined at 45xc2x0 relative to the polarization direction SC of the screen by the half-wave plates as illustrated in FIGS. 22A and 22B, or to convert the polarized beams into circularly polarized light beams by quarter-wave plates as illustrated in FIGS. 23A and 23B. FIG. 22A shows the relationship between the polarization directions of the beams (R, B, and G beams) emerging from the dichroic prism and the slow phase axis direction of the phase plates (indicated by the dashed line), and FIG. 22B shows the relationship between the polarization directions of the projected beams and the transmission-axis direction of the polarizing screen. FIG. 23A shows the relationship between the polarization directions of the beams (R, B, and G beams) emerging from the dichroic prism and the slow phase axis direction of the phase plates (indicated by the dashed line), and FIG. 23B shows the relationship between the polarization directions of the projected beams and the transmission-axis direction of the polarizing screen.
In such use ways, however, the intensity of the projected light decreases as follows because of absorption of light by a polarizing plate on the polarizing screen.
cos2(45)=0.5
Therefore, this poses another problem that brightness of the projected image becomes half, and the structure is not suitable for the image projection systems requiring the alignment of polarization directions.
If the polarization directions of the respective color beams incident to the dichroic prism are preliminarily aligned with each other there can be little loss of brightness at the polarizing screen. However, this will narrow the widths of the reflection and transmission bands of the dichroic films, as illustrated in FIG. 24, thus decrease margins for the wavelength components of the respective color beams transmitted or reflected by the dichroic prism, and increase the loss of light amount due to the incident angle characteristics of the dichroic films.
An object of the present invention is to provide a projection apparatus and an observation apparatus that can achieve higher utilization efficiency of light than the conventional apparatus.
A projection apparatus according to one aspect of the present invention is a projection apparatus comprising means for supplying light, a color separating system for separating the light from the means into a plurality of color beams, a plurality of light modulating elements for modulating the respective color beams separated by the color separating system, based on an image signal, a color combining system for combining the color beams emerging from the respective light modulating elements, and a projection optical system for projecting composite light of the color beams combined by the color combining system onto a plane, wherein the color combining system comprises a plurality of dichroic films, each of the color beams incident to the cross dichroic film is a linearly polarized light, and the following relation is met:
0xc2x0 less than xcex8 less than 90xc2x0
where xcex8 is an angle between a polarization direction of a color beam component transmitted by all the dichroic films and a polarization direction of a color beam component reflected by the dichroic film.
Another projection apparatus according to a further aspect of the present invention is a projection apparatus comprising means for supplying light, a color separating system for separating the light from the means into a plurality of color beams, a plurality of light modulating elements for modulating the respective color beams separated by the color separating system, based on an image signal, a color combining system for combining the color beams emerging from the respective light modulating elements, and a projection optical system for projecting composite light of the color beams combined by the color combining system, onto a polarizing screen, wherein the color combining system comprises a plurality of dichroic films, each of the color beams incident to the dichroic film is a linearly polarized light, and the following relation is met:
0xc2x0 less than xcex8 less than 90xc2x0
where xcex8 is an angle between a polarization direction of a color beam component transmitted by all the dichroic films and a polarization direction of a color beam component reflected by the dichroic film, and wherein a half-wave plate is placed in an optical path from the color combining system to the polarizing screen, and an angle between the polarization direction of the color beam component transmitted by all the dichroic films and a transmission polarization direction of the polarizing screen is substantially equal to an angle between the polarization direction of the color beam component reflected by the dichroic film and the transmission polarization direction of the polarizing screen.
In a preferred form of the above projection apparatus, the half-wave plate is provided at an exit side of a projection lens of the projection optical system.
In a preferred form of the above projection apparatus, the half-wave plate is provided between the color combining system and a projection lens of the projection optical system.
In a preferred form of the above projection apparatus, a slow phase axis of the half-wave plate rotates about the optical axis of the projection optical system.
Another projection apparatus according to a further aspect of the present invention is a projection apparatus comprising means for supplying a plurality of color light beams, a plurality of light modulating elements for modulating the respective color beams, based on an image signal, a color combining system which has a plurality of dichroic films for combining the color beams emerging from the respective light modulating elements, and a projection optical system for projecting composite light of the color beams combined by the color combining system onto a plane, wherein each of the color beams incident to the dichroic film of the color combining system is converted into linearly polarized light, and the following relation is met:
0xc2x0 less than xcex8 less than 90xc2x0
where xcex8 is an angle between a polarization direction of a color beam component transmitted by all the dichroic films and a polarization direction of a color beam component reflected by the dichroic film.
In a preferred form of the above projection apparatus, a half-wave plate is provided at an exit side of a projection lens of the projection optical system.
In a preferred form of the above projection apparatus, a half-wave plate is provided between the color combining system and a projection lens of the projection optical system.
In a preferred form of the above projection apparatus, a slow phase axis of the half-wave plate rotates about the optical axis of the projection optical system.
In a preferred form of the above projection apparatus, the polarization direction of the color beam component reflected by the dichroic film is s-polarized light to the dichroic films.
In a preferred form of the above projection apparatus, the angle e satisfies the following relation:
0xc2x0 less than xcex8 less than 80xc2x0.
In a preferred form of the above projection apparatus, the angle xcex8 satisfies the following relation:
xe2x80x830xc2x0 less than xcex8 less than 60xc2x0.
In a preferred form of the above projection apparatus, the angle xcex8 satisfies the following relation:
0xc2x0 less than xcex8 less than 45xc2x0.
In a preferred form of the above projection apparatus, the angle xcex8 satisfies the following relation:
xcex8=45xc2x0.
An observation apparatus according to a further aspect of the present invention is an observation apparatus with which an observer, wearing polarizing glasses to which light beams of polarization states different from each other are incident selectively to the left eye and to the right eye, observes a stereoscopic image from parallax images projected onto a polarizing screen, which preserves polarization directions, by first and second projection devices, wherein each of the first and second projection devices comprises means for supplying light, a color separating system for separating the light from the means into a plurality of color beams, a plurality of light modulating elements for modulating the respective color beams separated by the color separating system, based on an image signal, a color combining system comprising a cross dichroic prism with dichroic films on joint surfaces between four prisms, for combining the color beams emerging from the respective light modulating elements, a projection optical system for projecting composite light of the color beams combined by the color combining system, onto the polarizing screen, and a polarizer placed in an optical path from the cross dichroic prism to the polarizing screen, the polarizer having a polarization axis directed along a direction which bisects an angle between a polarization direction of a color beam component transmitted by all the dichroic films and a polarization direction of a color beam component reflected by the dichroic film, wherein each of the color beams incident to the dichroic film is a linearly polarized light, and the following relation is satisfied:
0xc2x0 less than xcex8 less than 90xc2x0
where xcex8 is the angle between the polarization direction of the color beam component transmitted by all the dichroic films and the polarization direction of the color beam component reflected by the dichroic film, and wherein a phase plate capable of altering a polarization state of light is set at an exit position of an image projection optical system of at least one of the first and second image projection devices, whereby polarization states of light beams projected from the two projection devices are made different from each other.
In a preferred form of the above observation apparatus, the angle xcex8 satisfies the following relation:
0xc2x0 less than xcex8 less than 80xc2x0.
In a preferred form of the above observation apparatus, the angle xcex8 satisfies the following relation:
0xc2x0 less than xcex8 less than 60xc2x0.
In a preferred form of the above observation apparatus, the angle xcex8xc2x0satisfies the following relation:
0xc2x0 less than xcex8 less than 45xc2x0.
In a preferred form of the above observation apparatus, the angle xcex8 satisfies the following relation:
74 =45xc2x0.
A system according to a further aspect of the present invention is a system for projecting a video picture by either of the projection apparatus described above.
A system according to a further aspect of the present invention is a system for projecting an image produced by a computer, by either of the projection apparatus described above.
In a preferred form of the above apparatus,
xcex8=80xc2x0.